The present invention relates generally to a radio communication system and, more specifically, to techniques of controlling the feedback operation of a user terminal in coordinated multi-point (CoMP) transmission/reception schemes.
Coordinated multi-point transmission/reception is considered in LTE (Long Term Evolution)-Advanced Release 11(Rel. 11) as a tool to improve the coverage of high data rates, the cell-edge throughput, and also to increase system throughput as described in the Sect. 4 of 3GPP TR 36.819 v11.0.0, Coordinated multi-point operation for LTE physical layer aspects (Release 11) (hereinafter referred to as “NPL 1”). The CoMP schemes, joint transmission (JT), dynamic point selection (DPS), and coordinated scheduling/coordinated beamforming (CS/CB) have been agreed to be supported as described in the Sect. 5.1.3 of NPL1. In case of JT and DPS, data for a user equipment (UE) is available at more than one point in the CoMP cooperating set, which is defined in the Sect. 5.1.4 of NPL1 as a set of (geographically separated) points directly and/or indirectly participating in data transmission to a UE in time-frequency resource. In case of CS/CB, data for a UE is only available at and transmitted from the one point (serving point) but user scheduling/beamforming decisions are made with coordinated among points corresponding to the CoMP cooperating set. It should be noted that the term “point” for coordinated multi-point transmission/reception can be used as a radio station, a transmission/reception unit, remote radio equipment (RRE) or distributed antenna of a base station, Node-B or eNB. Accordingly, in the present disclosure, a point, a radio station, a transmission/reception unit and a cell may be used synonymously.
For JT, multiple transmission points (TPs) are selected for simultaneous data transmission and the interference comes from the points other than the selected TPs. For DPS, only one TP is dynamically selected and the interference comes from the points other than the only selected TP. While, for CB/CS, the serving point is the only TP to transmit data but the strong interference from the neighbor cell is reduced significantly. Therefore, the employment of different CoMP schemes results in variable types of interferences.
It has been decided to standardize a common feedback framework that can support CoMP JT, DPS and CB/CS, as stated in the Sect. 5.2.2 of NPL1. For channel-dependent scheduling to support all the above CoMP schemes, CoMP measurement set is defined in the Sect. 5.1.4 of NPL1 as a set of points about which channel state/statistical information (CSI) related to their link to the UE is measured and/or reported, where the channel quality information (CQI) considering the interference power with muting on different cells in the CoMP measurement set needs to be estimated at UE side and fed back by the UE to the network.
In Rel. 8/9, cell-specific reference signal (CRS) is used for CSI measurement. The interference measurements are left completely unspecified with UE behavior. In Rel. 10, non-zero-power CSI-RS and zero-power CSI-RS have been introduced for CSI measurement of single cell downlink transmission as described in the Sect. 6.10.5.2 of 3GPP TS 36.211 v10.3.0, Physical Channels and Modulation of Evolved Universal Terrestrial Radio Access (E-UTRA) (Release 10) (hereinafter, referred to as “NPL2”). In Rel. 11 CoMP work item (WI), it has been proposed to use multiple non-zero-power CSI-RS over orthogonal resources to measure signals from multiple points in the CoMP measurement set.
Also it has been proposed to specify the interference measurement resources, such as zero-power CSI-RS in R1-114260, Ericsson, ST-Ericsson (hereinafter, referred to as “NPL3”) or PDSCH muting resource elements (REs) in R1-114228, Samsung (hereinafter, referred to as “NPL4”), to measure part of the intra- and inter-CoMP cluster interferences by configuring variable muting patterns for the points in the CoMP measurement set.
According to the Sect. 6.3.2 of 3GPP TS 36.331 v10.3.0, Radio resource control (RRC) and Protocol specification of Evolved Universal Terrestrial Radio Access (E-UTRA) (Release 10) (hereinafter, referred to as “NPL5”), the serving point semi-statically informs the UE of only the resource location, i.e., the resource element (RE) index and subframe index, of non-zero-power CSI-RS and zero-power CSI-RS as the information element of CSI-RS-Config by radio resource control (RRC) signaling. For multi-point signal power measurement by using multiple non-zero-power CSI-RSs, the additional information, such as the cell identity (ID) of each cell in the CoMP measurement set, may also be needed for UE detection, when non-zero-power CSI-RSs are generated by using cell-specific scrambling sequences as stated in R1-114318, Nokia Siemens Networks, Nokia (hereinafter, referred to as “NPL6”), where the physical cell ID is used to generate the initialization value of the cell-specific scrambling sequence, as defined in the Sect.6.8.2 of NPL2.
In order to explain problems to be solved by the present invention, a simple example for a system with the employment of CoMP is given in FIG. 1. Referring to FIG. 1, it is assumed that a UE is served by Cell1 and a CoMP measurement set is composed of the serving cell (Cell1) and cooperating cells (Cell2 and Cell3). The Cell1, Cell2 and Cell3 are managed by a central control unit (CCU) through backhaul links BL1, BL2 and BL3, respectively.
As shown in FIG. 2A, the CCU configures orthogonal non-zero-power CSI reference signals CSI-RS1, CSI-RS2 and CSI-RS3 for Cell1, Cell2 and Cell3, respectively. Moreover, the zero-power CSI-reference signals ZP-CSI-RSa, ZP-CSI-RSb, ZP-CSI-RSc and ZP-CSI-RSd are also configured to measure the interference with muting on different cells, where the muting patterns on different zero-power CSI-RS resources are indicated by Ma, Mb, Mc and Md, respectively. In the muting pattern Ma, the signal power of Cell1 and Cell2 are muted; in the muting pattern Mb, the signal power of Cell1 and Cell3 are muted; and in the muting pattern Mc, the signal power of Cell2 and Cell3 are muted; in case of Md, the signal power of Cell1˜3 is muted. When the signal power of a Cell is muted, the interference from that Cell can be eliminated from consideration about interference at UE. Accordingly, in the muting pattern Md, the interferences within the CoMP measurement set are all excluded.
As shown in FIG. 2B, the UE is informed of configuration information of CSI-RS1, CSI-RS2 and CSI-RS3 including their resource positions defined by RE index and subfame index as well as the corresponding cell ID. Based on the configuration information, the UE can measure the signal power per cell: S1, S2 and S3. Furthermore, with the knowledge of the resource positions of zero-power (ZP)-CSI-RSa, ZP-CSI-RSb, ZP-CSI-RSc and ZP-CSI-RSd, the UE can calculate the interference power Ia, Ib, Ic, Id according to the muting patterns Ma, Mb, Mc and Md by measuring and averaging the received power at the informed positions of ZP-CSI-RSs as follows:                Muting pattern Ma: Ia=average(0+0+S3+N)        Muting pattern Mb: Ib=average(0+S2+0+N)        Muting pattern Mc: Ic=average(S1+0+0+N)        Muting pattern Md: Id=average(0+0+0+N)where N represents the interferences from outside the CoMP measurement set plus the additive white Gaussian noise (AWGN).        
Thereafter, the per-CSI-RS-resource CQI considering muted interferences for CoMP can be calculated at the UE as shown in FIG. 2(C).
Since the network informs the UE of only the RE positions of CSI-RSs, the UE cannot know which cell's signal is counted for CQI calculation and therefore uses all interferences to calculate CQI as shown in FIG. 2C. For example, the signal power S1 for Cell1 measured at the UE is not muted in the muting pattern Mc and therefore the calculation Ic=aver(S1+N) does not reflect actual interference at the resource of CSI-RS1.
However, since the UE is informed of only the RE positions of CSI-RSs, the calculation Ic=average(S1+N) is assumed to be counted, resulting in a wrong CQI, such as S1/Ic. Similar case would happen if S2/Ib=S2/average(S2+N) and S3/Ia=S3/average(S3+N) are measured and reported as shown in FIG. 2(C). The calculation and feedback of a wrong CQI may result in useless power consumption and unnecessary feedback overhead.
An object of the present invention is to provide a method and system which allow efficient measurement and feedback of CQI for CoMP scheduling.